How old is the Earth?

Ancient rocks exceeding 3.5 billion years in age are found on all of Earth's continents. The oldest rocks on Earth are the Acasta Gneisses in northwestern Canada near Great Slave Lake (4.03 billion years) and the Isua Supracrustal rocks in West Greenland (3.7 to 3.8 billion years), but well-studied rocks nearly as old are also found in the Minnesota River Valley and northern Michigan (3.5-3.7 billion years), in Swaziland (3.4-3.5 billion years), and in Western Australia (3.4-3.6 billion years).

Mineral grains (zircon) from sedimentary rocks in west-central Australia, which have been radioactively dated using the Uranium-Lead method, yield ages of 4.4 billion years and are the oldest remnants of Earth's first crust. The oldest dated moon rocks have ages between 4.4 and 4.5 billion years and provide a minimum age for the formation of our nearest planetary neighbor. Thousands of meteorites, which are fragments of asteroids that fall to Earth, have been recovered.

The results show that the meteorites, and therefore the Solar System, formed between 4.53 and 4.58 billion years ago. A consistent age for the whole shebang is therefore closer to 4.54 billion years with an uncertainty of less than 1 percent. The Moon formed about 100 million years later.

This answer was updated in 2011. See my books: The Astronomy Cafe (1998) and Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011

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